This application proposes to follow serologically embryonic cell surface antigens in the mouse that are characteristic of specific early stages of cell differentiation both in the embryo and on embryonal tumor cells, specifically teratocarcinoma and a genetically caused medulloblastoma-neuroblastoma. The project will make use of a series of mutant genes at the T-locus which are known to: 1) interfere with differentiation at specific times in development, 2) specify cell surface antigens on sperm and in the embryo, and 3) in the one case studied, cause an embryonal tumor. We would like to know the time course of the expression of these antigens, if they are functional, and what role the mutant form of the antigen plays in malignant differentiation. These objectives involve transplantation of mutant embryos, obtaining transplantable tumors from t-mutant cells, and serological definition of embryonic surface antigens by means of cytotoxicity tests and absorption studies.